Transcript Acid Mine Drainage - University of Colorado Boulder

Acid Mine Drainage
Terms
• Acid Mine Drainage (AMD)
– Water that is polluted from
contact with mining activity
• Acid Rock Drainage (ARD)
– Natural rock drainage that is
acidic
• Both produce acidic waters
• How to distinguish?
Characteristics
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Increased acidity = decreased pH
Increased metal concentrations
Increased sulfate
Increased suspended solids
All four don’t necessarily occur at the
same time
Stream Effects
• Colored waters:
• “Yellow boy”
– Iron oxides, basically
rusting the stream floor
• White
– Aluminum
• Black
– Manganese
• Determined by shifts in
pH
Shift in Mining Techniques
• “Old school”
– Abandoned mines
– Tailings/waste rock piles
– ARD
• “New School”
– Cyanide heap leach mining
AMD Chemistry
• Pyrite weathering
pyrite
water
+
air
low pH
+ metals
AMD Chemistry
Iron oxide
4FeS2 + 14 H2O + 15 O2 → 4Fe(OH)3 + 8 SO42- + 16 H+
Overall acid producing
AMD Chemistry
• Surface area
– more surface area, faster rate
– smaller grains, more surface area
Extent of Problem
• Colorado
– 20,000+ mines
– 1,300 miles of streams
• Montana
– 20,000+ mines
– 1,000 miles of streams
• Arizona
– 80,000+ mines
– 200 miles of streams
Treatment
• Active v. Passive
• Active
– physical addition of alkalinity to raise pH
– High cost
– effective
• Passive
– Naturally available energy sources
– Little maintaince
– Driven by volume
Passive Treatment
Active Treatment
Typical treatment processes (“ODAS”)
-oxidation
-dosing with alkali
-sedimentation
Active Treatment
Iron Mountain, California
“New School”
• Cyanide Heap Leach
– Extract gold from low grade ore
– Ore crushed, placed in open air leach
pads
– Cyanide sprayed on top
– Leaches gold as migrates through ore
– Solution drained, gold recovered
– Pretty huh?
Summitville Mine
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Rio Grande Headwaters
Elevation 12,800’
Snowfall: 7-11 m/ year
Population: 700
112 stamping machines
Abandoned in early 1900s
– Gold prices fell, diminishing returns, weather
issues
Summitville
• 1984
– Application for mining permit
• 1985
– Large scale open pit gold mine
– Cyanide leaching
• 1986
– Construction. Problems.
– HDPE liner ripped during placement, but not
fixed.
Summitville
• 1987-1991: Heap Leach Pad
– 73 acres
– One pile >190’
– No outlet for water, only lost through ET
– Snowfall underestimated, ET overestimated
Now What?
Summitville
• 1987-1991 cont
– Permit to discharge
excess water. Limits in
concentrations
– Could not meet limits
– Fish kills downstream
for 17 miles in Alamosa
River
Summitville
• 1992
– EPA assumes control, $20,000,000 to ‘fix’
– Heap leach pad near overflow, discharging 3,000
gallons/minute through leaks
– 200 million gallons of cyanide laced water
– Not last till spring snowmelt
Costs
• To date: $185 million
– Annually: $1.5 million
• Taxpayers foot bill
• Mine owner cost: $3 million bond
SUMMARY
• 4 characteristics of AMD
• Pyrite weathering (Fool’s gold) source of
acidity in many AMD problems
• “Old School” and “New School” mining
• Cyanide heap leach technology
• Summitville example